1. Field of the Invention
The present invention relates to a centrifugal pump. More particularly, the present invention relates to a centrifugal pump which is mounted on a vehicle for assisting circulation of liquid.
2. Description of the Related Art
In recent years, the number of vehicles having an “idle stop” function has been increasing for helping stop global warming. The idle stop function is turning off an engine when a vehicle is stopped, for example, at a red light, so as to reduce emissions. The idle stop function also makes vehicles more efficient.
In vehicles using a re-heat type air conditioning system, however, when the engine is stopped by the idle stop function, hot coolant from the engine is not delivered to a heater core. This may lower a heating performance of the air conditioning system. In order to avoid this problem, vehicles are usually equipped with an electric pump which operates to circulate coolant when the engine is stopped.
That electric pump does not operate while the engine is operating, but forms a portion of a coolant passage from the engine to the heater core. Thus, an impeller of the electric pump may interfere with a coolant flow in the passage if the impeller has a particular shape. In this case, flowing resistance in the coolant passage from the engine to the heater core, especially inside the electric pump, is increased, and may lower flow efficiency of coolant from the engine to the heater core. In particular, when a passenger rides in a vehicle, the flow resistance in a case where the electric pump forms a portion of the coolant passage is important because a period during which the engine is operating is longer than a period during which the engine is stopped.